Femtoamp Current Preamp for Microprocessor Controlled Isotope Calibrator

Currents as low as ten femtoamps (10-14amps) are accurately measured in an isotope calibrator design employing a highly sensitive, highly stable current integrating preamp. This current corresponds to 0.2 microcuries of 57Co irradiating a typical Argon filled chamber at 2 atmospheres. The preamp design is discussed, primarily with respect to leakage currents, bias stability, bias memory for a 10 second integration, low frequency noise, and dynamic range. Results of the preamp performance are presented for the isotope calibrator application, and the role of the microprocessor is discussed briefly.